1. Field of the Invention
The present invention relates to a liquid crystal display device which can be used, for example, for a TV screen, various office automation instruments and other display panels and a method for preparing the liquid crystal display device.
2. Description of the Related Art
Hitherto, a liquid crystal display device has been formed by injecting a liquid crystal material between a pair of transparent electrodes which are fixed with a gap of several micrometers. However, the above structure cannot give the preparation of the display having a large area.
In addition, brightness of the screen and an angle of view field are insufficient, since it is necessary to attach polarization plates having polarization axes which are perpendicular to each other to a pair of glass substrates enclosing the liquid crystal material.
In addition, except one using a ferroelectric liquid crystal, in the conventional liquid crystal display device, an orientated state has no memory so that an active matrix driving such as TFT which is produced in a low yield should be used to produce a display screen having a large number of picture elements. Therefore, the production cost increases. When the ferroelectric liquid crystal is used, a very thin cell gap control of 1 to 2 .mu.m and a uniform orientation of liquid crystal molecules are necessary, so that the ferroelectric liquid crystal cannot provide a satisfactory display even in a small area.
Recently, a new liquid crystal display device has been developed (cf. Japanese Patent Kokai Publication Nos. 193115/1990 and 127494/1990, Chem. Lett., 817 (1989) and Polymer Preprints, Japan Vol. 39, No. 8, 2372 (1990)). This device is prepared by casting and coating a solution of a polymer having a side chain of a structure corresponding to a liquid crystal compound and a conventional low molecular weight liquid crystal in a solvent on a plate-form support or a film such as a transparent electrode, drying and solidifying the solution to form a mixture film of the polymer liquid crystal and the low molecular weight liquid crystal and placing another support thereon. In this system, a side-chain type polymer liquid crystal and a low molecular weight liquid crystal form a homogeneous mixture and are not phase-separated.
In the above liquid crystal display device, when a low frequency or a direct current is applied on the mixture film, ions move in accordance with an electric field in the mixture film so that the orientation of the liquid crystal molecules is disturbed and the film becomes opaque. When a high frequency is applied on the mixture film, the liquid crystal molecules in the film are homeotropically orientated by an electro-optical effect, so that an incident light passes through the film without being scattered and the film becomes transparent. A polymer liquid crystal or a polymer chain is essential for this display mode. Without a polymer liquid crystal or a polymer chain, the mixture film does not change to an opaque state.
In this liquid crystal display device, after the electric field is removed, the display device has memory and maintains scattered or non-scattered states of light stably. In addition, since this liquid crystal display device is easily prepared by coating the solution of the polymer liquid crystal and the low molecular weight liquid crystal and drying it to form the mixture film of the polymer liquid crystal and the low molecular weight liquid crystal, a liquid crystal display having a large area is easily prepared.
However, since the conventional mixture film of the polymer liquid crystal and the low molecular weight liquid crystal contains a single low molecular weight liquid crystal, a temperature range of a smectic state which has memory is narrow so that the liquid crystal display does not work at room temperature.
Polymer Preprints, Japan, Vol. 39, No. 8, 2373 (1990) discloses a system containing two kinds of low molecular weight liquid crystals, one of which substitutes a mesogen group of the polymer liquid crystal. Therefore, a content of the polymer liquid crystal in the mixture film is decreased, so that the mixture film is soft, loses a self-supporting property and cannot be practically used. Since the mixture film is not self-supporting, a flexible liquid crystal display device cannot be prepared using such film, or a short-circuit is easily formed between the electrodes.